Let S be the set of all non-zero divisors of R and let T = {t ∈ S : tm = 0 for some m ∈ M
implies m = 0}. Let RT be the localization of R at T. For any non-zero submodule N of M,
let N−1 = {x ∈ RT : xN ⊆ M}. It is easily seen that N−1 is an R-submodule of RT , R ⊆ N−1
and N−1N ⊆ M. Following [10], N is an invertible submodule of M if N−1N = M. Following
[10], an R module M is called a Dedekind module (Prüfer module) if every non-zero (finitely
generated) submodule of M is invertible. Dedekind modules and Prüfer modules have been
extensively studied in [1] and [10].